1. Field of Invention
The present invention relates to a projection display apparatus having an optical system in which an optical image corresponding to image information is formed by optically processing a light beam emitted from a light source, and is enlarged and projected by a projection lens.
2. Description of Related Art
A projection display apparatus is known which comprises an optical system that forms an optical image corresponding to image information by optically processing a light beam emitted from a light source, and enlarges and projects this optical image using a projection lens, a power supply supplying electric power driving the device, an outer casing housing this optical system and the power supply, and a plurality of circuit substrates controlling the optical system.
The optical system in the projection display apparatus normally comprises a color separation system separating light from the light source into light beams of the three primary colors, an optical modulation device emitting the separated light beams as light beams modulated according to image information, and a color synthesizing system synthesizing the respective modulated light beams.
The color separation system has three dichroic mirrors. These dichroic mirrors are housed and arranged together with the light source inside a light guide that serves as a optical component housing and that ensures a predetermined optical path. A light beam from the light source is separated into the three primary colors R, G, and B by the three dichroic mirrors. The optical modulation device has three liquid crystal light valves corresponding to the separated light beams. The separated light beams are respectively modulated by these three light valves according to image information, and are emitted as modulated light beams. The color synthesizing system is constructed by a prism unit that synthesizes the modulated light beams and emits the synthesized light to the projection lens.
In such a projection display apparatus, a cooling mechanism is incorporated to cool the light source, the power supply, the optical system, and the circuit substrates that are heat sources. More specifically, air cooling is introduced from an air inlet formed in the outer casing by a suction fan, is passed via the heat sources disposed inside, and is then exhausted outside from an air outlet formed in the outer casing by an exhaust fan.
Since the light valves constituting the optical system, among the above-mentioned heat sources, are less resistant to heat than the other heat sources, they need to be cooled promptly. For this reason, the conventional cooling mechanism has a structure in which the air inlet of the outer casing is formed below the light valves and the prism, and in which cooling air is forcibly blown against the light valves and the prism by the suction fan near the air inlet.
In addition, since the cooling air is blown against the light valves, the prism, or the like, in the form of a swirling vortex, the light valves, the prism, or the like are not uniformly cooled. Furthermore, the vertical cooling air tends to spread outside the vortex, and therefore, it is difficult to guide the entirety of the cooling air to the light valves, the prism, or the like, which lowers the cooling efficiency for the light valves, the prism, or the like.
On the other hand, when cooling air is drawn unaltered into the inside of the device, dust floating in the air outside is also sometimes drawn into the inside of the device. When this dust adheres to the respective components of the optical system, the image quality of a projection image is deteriorated. In particular, dust is apt to adhere onto the surface of the light valve, which constitutes the optical modulation device, because the surface is electrically charged, and this is a great factor in deterioration of image quality. Furthermore, since the prism unit constituting the color synthesizing system is disposed immediately in front of the projection lens that enlarges and projects an image, adhesion of dust to the prism unit is also a great factor in deterioration of image quality, in a manner similar to the light valve.
For this reason, dust has been prevented from entering the inside of the device by covering the air inlet with an air filter cover having a spongy air filter.
In such a conventional dust-resistant structure, however, the inside of the device is uniformly protected from dust by the air filter. Therefore, it is difficult to reliably protect the optical modulation device and the color synthesizing system, which have significant effects on image deterioration, separate from other sections inside the device.